WiMAX, which stands for the Worldwide Interoperability for Microwave Access, is a standards-based broadband wireless technology that provides high-throughput broadband connections over long distances. There are two main applications of WiMAX today: fixed WiMAX and mobile WiMAX. Fixed WiMAX applications are point-to-multipoint, enabling broadband access to homes and businesses. Mobile WiMAX is based on OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) and offers full mobility of cellular users at broadband speeds.
In the mobile WiMAX standard, a mobile station (MS) may be allocated with a scanning interval in which the MS may halt its normal operation and scan the neighboring base stations (BS). The MS may also be allocated with an unavailable interval to power down the hardware in sleep mode. During the scanning interval or the unavailable interval, a BS may neither transmit to the MS nor expect to receive any messages from the MS.
A Hybrid Automatic Repeat-Request (HARQ)-enabled MS transmits uplink (UL) HARQ data bursts and receives downlink (DL) Acknowledgements (ACKs) from the BS. Similarly, the HARQ-enabled MS receives DL HARQ data bursts and transmits UL ACKs to the BS. In the WiMAX standard, the delay for the BS to send the DL HARQ ACK after receiving a UL HARQ data burst may be constant and may be given in a downlink channel descriptor (DCD) message. Similarly, the delay for the BS to receive a UL HARQ ACK message after transmitting a DL HARQ data burst may be constant and may be configured in an uplink channel descriptor (UCD) message. For example, both DL ACK and UL ACK delays may be set to 1, 2 or 3 frames after data burst transmission.
If a HARQ data burst is sent right before the start of a scanning interval or an unavailable interval of an MS, a corresponding HARQ ACK message may not be transmitted or received properly if it falls in the middle of the scanning interval or the unavailable interval. This may result in unnecessary retransmissions of data that was successfully received.